1. Field of the Invention
This invention relates to coordinate input devices such as touch keys that detect input positions manually designated on boards or panels associated with liquid crystal displays, so that corresponding coordinates are produced and input to electronic devices.
2. Description of the Related Art
Liquid crystal displays are widely used for a variety of small-size portable electronic devices such as digital clocks, watches, calculators, electronic pocketbooks, and PDA (abbreviation for ‘Personal Digital Assistant’) devices. A typical example of the small-size portable electronic device provides a liquid crystal display that has a touch panel function allowing entry of data and switching of functions by detecting touch or contact thereon. For example, the liquid crystal display is capable of displaying various pieces of information representing calendars, schedules, directories, addresses, and residences, which should be adequately managed on the screen thereof In addition, it also displays function buttons or areas, which are used to switch over functions when operated. That is, when a user or human operator touches the prescribed function area with his/her finger or an electronic pen on the screen, the electronic device changes the images on the screen in accordance with the designated function. Some electronic devices also allow entry of handwritten letters or pictures on the screen. This type of the portable electronic device provides a specifically designed liquid crystal display that has touch keys attached onto its panel surface or substrate, wherein these touch keys act as switches for changing over functions or input devices for inputting coordinates.
Conventionally, various types (of coordinate input devices such as touch keys were actually produced for practical uses. That is, the coordinate input devices were designed based on the resistance film method, electrostatic capacitance method, ultrasonic method, and the like. In the case of the coordinate input device of the ultrasonic method, for example, the user holds a position input member such as an electronic pen incorporating a vibration generator, and brings it in contact with a board or substrate. The board provides vibration detectors such as piezoelectric elements, which detect vibration propagating thereto from the position input member whose tip is brought in contact with the prescribed position. Thus, the coordinate input device estimates the ‘contact’ position (representing coordinates, etc.) based on the vibration propagating time.
The coordinate input device of the resistance film method provides a relatively low transmittance, which may deteriorate visibility of the display device. The coordinate input device of the electrostatic capacitance method requires an electronic pen specially designed for designation of coordinates. In addition, both the resistance film method and electrostatic capacitance method basically require electrodes for the purpose of detection of electric current and capacitance. Therefore, the coordinate input devices of these methods are complicated in manufacturing processes. Further, the coordinate input device of the ultrasonic method also requires a specially designed electronic pen incorporating a vibrator for generating vibration.